1. Field of the Invention
The present invention relates to a light emitting diode lamp, and more particularly to a light emitting diode lamp that has low resistance to heat radiated therefrom.
2. Discussion of the Background
Light Emitting Diode (LED) lamps can realize various colors and are widely used for display lights, electronic display boards, and display devices. Further, the LED lamps are is also applied to general lighting based on characteristics of realizing white light. The LED lamps have various merits, such as high efficiency of light emission, long service life, and environmentally friendliness, which enable a continuous increase in application of the LED lamps to various fields.
FIGS. 1 (a) and (b) are a front view and a plan view of a conventional LED lamp, respectively.
Referring to FIG. 1, the conventional LED lamp includes a first lead 1 which has a top part 3 and a leg extending from the top part 3. A second lead 2 is spaced apart from the first lead 1. The second lead 2 has a leg corresponding to that of the first lead 1 and a top part adjacent the top part 3 of the first lead 1.
An LED 5 is mounted on the top part 3 of the first lead 1 and is electrically connected to the top part of the second lead 2 via a bonding wire 7. Generally, the top part 3 of the first lead has a cavity formed to receive the LED 5 therein. A side wall of the cavity constitutes a sloped reflection surface to reflect light emitted from the LED 5 in a predetermined direction.
A transparent encapsulating material 9 surrounds the top parts of the first and second leads 1 and 2 and LED 5. The transparent encapsulating material 9 is formed of a silicone or epoxy resin which transmits light emitted from the LED 5. On the other hand, a curable resin containing a phosphor may cover the LED 5 within the cavity.
FIG. 2 shows a lead frame used for the LED lamp shown in FIG. 1.
Referring to FIG. 2, an alloy plate having a predetermined thickness is punched to form first leads 1 and second leads 2, each of which has a top part and is supported by a support frame 11. The first and second leads 1 and 2 are arranged on the same plane.
LEDs 5 are mounted on the top parts of the first leads 1 and electrically connected to the second leads 2 via bonding wires 7. Then, the encapsulating material 9 (see FIG. 1) is prepared to cover the LEDs 5 and the top parts of the first and second leads 1 and 2. Generally, the encapsulating material 9 is formed by disposing the first and second leads 1 and 2 upside down in a mold containing a liquid or gel epoxy resin, followed by curing the epoxy resin. Before forming the encapsulating material 9, a curable resin may be dotted on the LEDs 5.
Then, the first and second leads 1 and 2 are separated from the support frame, thereby completing individual LED lamps.
FIG. 3 is a partially enlarged view of the lead frame illustrating problems of the conventional LED lamp.
Referring to FIG. 3, the top part 3 of the first lead 1 has a height (H) and the top part of the second lead 2 is formed with a plane which faces the top part 3 of the first lead 1 and has a length (L). The plane having the length (L) is separated a distance (δ) from the top part of the first lead 1. Since the encapsulating material 9 (see FIG. 1) surrounds the top parts of the first and second leads 1 and 2, the encapsulating material 9 is disposed between the top parts of the first and second leads 1 and 2.
Such a conventional LED lamp has many problems relating to thermal resistance.
First, since the encapsulating material 9 is formed of only a transparent material, opaque materials, such as ceramics or plastics, which have high thermal conductivity and are used for general semiconductor packages, cannot be applied to the encapsulating material 9. Thus, there is a limit in dissipating heat through the encapsulating material 9.
Second, since the first lead 1 has the very thin and long leg, there is a limit in dissipating heat through the leg. Additionally, although heat can be transferred to the second is lead through the bonding wire 7, there is a limit in heat transfer due to the thinness and length of the bonding wire 7.
Third, there is a limit in heat transfer from the top part 3 of the first lead 1 to the adjacent top part of the second lead 2. In other words, the top part of the second lead 2 adjacent to the top part 3 of the first lead 1 is relatively narrow, whereas a distance between the top parts is significantly wide. Further, since the transparent encapsulating material 9 having a lower thermal conductivity is disposed between the top parts, there is a limit in heat transfer from the top part 3 of the first lead 1 to the top part of the second lead 2.
Various attempts have been made to lower the problematic thermal resistance of the conventional LED lamp. FIG. 4 shows conventional lead frames of LED lamps designed to reduce the thermal resistance.
In a lead frame shown in FIG. 4 (a), a first lead 21 has a rectangular top part 23 which has an increased height (H). With this configuration, the top part 23 of the first lead 21 has enhanced thermal capacity, and adjacent sections between the top part 23 of the first lead 21 and a top part 22a of a second lead 22 are increased, thereby reducing the thermal resistance.
Simulation with a heat transfer coefficient of 40 W/m2K in still air, surrounding temperature of 0° C., and power of 1 W applied to an LED chip showed that height increases to 1 mm, 2 mm, and 3 mm resulted in thermal resistances of 87° C./W, 79° C./W, and 73° C./W, respectively.
In a lead frame shown in FIG. 4 (b), like the first lead 21 of FIG. 4 (a), a first lead of this lead frame has a rectangular top part 33 which has an increased height (H) to increase a surface area of the top part 33 adjacent to a top part 32a of a second lead. Additionally, ears are respectively provided to the top parts 33 and 32a of the first and second leads to increase thermal is capacity of the top parts 33 and 32a, thereby lowering the thermal resistance.
In FIG. 4 (a), when the height (H) of the top part is 3 mm, the thermal resistance was reduced to 71° C./W by addition of the ears.
In a lead frame shown in FIG. 4 (c), top parts 43 and 42a of first and second leads are formed with ears like the lead frame shown in FIG. 4 (b), but have a trapezoidal shape to further increase surface areas of adjacent planes. Accordingly, heat transfer from the top part 43 of the first lead to the top part 42a of the second lead can be promoted, thereby lowering the thermal resistance.
According to the conventional techniques, the thermal resistance can be lowered to some degree by increasing the height of the top part of the first lead, forming the ears or increasing the surface areas of adjacent planes between the top part of the first lead and the top part of the second lead. However, increasing the height of the top part of the first lead, forming the ears or increasing the surface areas of adjacent planes between the top part of the first lead and the top part of the second lead cannot achieve sufficient reduction in thermal resistance due to the size of the LED lamp.
Therefore, there is a need for a new LED lamp that can lower the thermal resistance by facilitating heat transfer from a top part of a first lead to other parts.